Method and device for charging an electrical accumulator battery by means of solar-cells

ABSTRACT

The invention relates to a method of charging an electrical accumulator battery (16) by means of solar panels (11-14) with solar-cells. The battery (16) is charged by means of two or more solar panels (11-14) included in charging branches connected in parallel, at least one charging branch being permitted to charge the accumulator battery to a higher voltage than the other charging branch or branches. 
     Only the charging branch or branches which charge to the highest voltage are permitted to deliver current to the acccumulator battery (16) during maintenance charging, when the accumulator battery is fully charged. 
     A device for carrying out the method consists of charging branches (11-21, 12-22 , 13-23, 14-24) comprising at least one solar-cell panel and connected in parallel, the charging branches being adapted to deliver charging current at different voltages. At least one voltage is permitted to be higher than the other or others. 
     The charging branch (14-24) which delivers the highest voltage is adapted to deliver a voltage which is somewhat higher than the voltage of the fully charged accumulator battery.

TECHNICAL FIELD

The present invention relates to a method and a device for charging anelectrical accumulator battery by means of solar-cell panels. Suchpanels consist of a plurality of solar-cells which are coupled in seriesand/or in parallel so as to obtain a suitable voltage and current fromthe solar-cell panel.

PRIOR ART

Energy generators driven by solar-cells consist usually of a solar panelfrom which the generated current is conveyed to a battery to charge it.There is a charging regulator which, with a low terminal voltage at thebattery during the charging, permits a higher current to charge thebattery while, at the maximum voltage of the battery, the chargingcurrent is very low and serves as a maintenance current to compensatefor self-discharge of the battery. A load is connected via the battery.When the load is connected and the battery is discharged, the terminalvoltage drops. The charging regulator then regulates voltage and currentfrom the solar panel to a suitable value in relation to the state of thebattery.

Solar energy generators with solar-cells have manifold applications aspower sources. It is typical of the majority of applications that thedriven devices or measuring instruments are situated in remote andisolated places where no electrical energy is available. The size of thesolar panels and batteries naturally varies within wide limits dependingon the required installation. Examples of such unmanned installationswhere solar energy is used are telephone communication in the microwaverange, slave stations for VHF radio, emergency radio transmitters,signals and safety equipment on railways, weather stations and otherremotely situated measuring stations, navigation aids and otheroff-shore equipment, fire protection devices and fire alarms, cathodeprotection, electric fences etc.

Solar energy devices can be expected to work at extreme temperatures,both high and low. Despite this, the installation must be so reliable inoperation that intervals in maintenance go up to a year or more.

A major problem is that the battery may be exposed to powerfulovercharging when it has reached its fully charged state with maximumterminal voltage. The charging current then causes decomposition of thewater in the electrolyte, which leads to the water being carried away.In hot regions, in in particular, this is a problem since thetemperature is normally so high that a certain evaporation occurs. It istherefore necessary to regulate the charging voltage so that onlyinsignificant overcharging occurs at the maximum terminal voltage of thebattery. Heavy demands are thus imposed on the operational reliabilityand life of the charging regulator.

THE INVENTION

The object of the invention is to achieve rapid charging of batteries bymeans of high current intensities while at the same time the risk ofwater losses and disturbances in operation is minimized so that longservice intervals are achieved, besides which the method discovered andthe device intented have an astonishingly low cost.

While high current intensities can be caused during charging accordingto the invention, at the same time very low current intensities can alsobe used, particularly when the battery consists of nickel-cadmium cells.

This is brought about as a result of the fact that the battery ischarged by means of two or more solar panels included in chargingbranches connected in parallel and having substantially the same power,one or more diodes being connected in series between at least one of thesolar panels and the battery so that at least one charging branch ispermitted to charge the accumulator battery to a higher voltage than theother charging branch or branches.

As a result, a high current intensity is obtained at the beginning ofthe charging and a low current intensity at the end of the chargingwhich does not cause any appreciable decomposition of the water.

It is also possible to charge the battery as a result of the fact thatonly the charging branch or branches which charge to the highestvoltages are permitted to deliver current to the accumulator batteryduring maintenance charging, when the accumulator battery is fullycharged.

When the battery is partially discharged or when it is loaded, aplurality of charging branches can be permitted to deliver current tothe accumulator battery.

The method invented may appropriately be carried out by means of aspecial device for charging an electrical accumulator battery by meansof solar-cells containing solar panels with charging branches comprisingat least one solar-cell panel connected in parallel, the chargingbranches being adapted to deliver charging current at differentvoltages, and at least one voltage being permitted to be higher than theother or others.

According to one embodiment of the invention, the charging branch whichis adapted to deliver the highest voltage is adapted to deliver avoltage which is somewhat higher than the voltage of the fully chargedaccumulator battery.

According to another embodiment, all the charging branches are adaptedto deliver successively higher voltages.

In another embodiment of the invention, a change-over switch may beprovided to connect one or more diodes in series between one or morepanels and the battery or a control device for each change-over switchfor the automatic connecting of the diodes depending on the chargingvoltage of the battery.

With a device according to the invention, the water loss in the batterycan be restricted to a minimum. As an example, it may be mentioned that8 hours solar radiation per day, which can be regarded as a normalmaximum, with a certain capacity of the solar panel corresponds to about1460 Ah overcharging per annum and a corresponding water loss of 487 ml.A standard nickel-cadmium battery at 200 Ah may have an electrolytereserve of 700 ml. Therefore under difficult conditions, such a batterycan be used for more than 1.5 years without refilling.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in more detail with reference to theaccompanying drawings in which:

FIG. 1 shows a conventional solar energy generator,

FIG. 2 shows the relation between the voltage and current of asolar-cell panel with constant irradiation of solar energy,

FIG. 3 shows the necessary charging voltage for charging a battery,

FIG. 4 shows a device according to the invention, and

FIG. 5 shows an alternative embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1 a solar panel with solar-cells is designated by 1, whereas 2is a possible charging regulator and 3 is a blocking diode. The batteryis designated by 4 and the load or device driven by the battery and thesolar-cell panel by 5.

FIG. 2 shows the electrical characteristics for the relation between thevoltage U and the current I which are delivered by a solar-cell panelwith constant solar irradiation. The Figure shows successive voltages V₁-V₄ and the associated current values I₁ -I₄. The maximum power P_(max)is obtained at a point where the curve begins to incline downwards andthe corresponding current and voltage values are I_(max) and V_(max). Itis naturally desirable that the solar-cells should work with the maximumpower P_(max). This can be achieved by series and parallel connection,in a suitable manner, of indivudal solar-cell elements.

FIG. 3 shows the charging voltage U of the battery 4 depending on thecharging time T. In a completely or partially discharged state, thevoltage is low rising to the maximum terminal voltage of the battery atthe end of the charging. At the end of the charging, the chargingvoltage rises steeply and then flattens out.

In order to utilize the output of the solar-cells in the best manner,the number of cells in the accumulator battery which is to be chargedshould be selected so that the charging voltage of the battery is aboutV_(max). If the number of cells in the battery is selected so that thevoltage at the beginning of the charging of a discharged battery isV_(max), the current drops to a great extent in the course of thecharging and the energy of the solar-cells is poorly utilized and thecharging of the battery becomes incomplete. On the other hand, if thenumber of cells is selected so that V_(max) is only reached at the endof the charging, the whole current I_(max) is supplied to a large extentto the battery for maintenance charging, in which case the waterconsumption becomes troublesome.

FIG. 4 shows one embodiment of the invention. In order to charge thebattery in a relative short time without causing unacceptable waterconsumption, in the solar energy generator shown, four similar standardsolar-cell panels 11, 12, 13 and 14 are connected in parallel betweentwo outging conductors 15 and 25. In the example shown, three standarddiodes 21 are connected in series one after the other in the outgoingcircuit of the panel 11. The solar-cell panel 12 has two standard diodes22 in its outgoing circuit, the panel 13 has one diode 23 and the panel14 is directly connected to the battery.

As mentioned earlier, the number of cells in the battery is selected sothat the battery voltage at full charge is close to V_(max), see FIG. 2.If the battery is not fully charged, its voltage is so low, underV_(max) in FIG. 2, that all the panels deliver substantially equallyheavy currents, as a result of which the total charging current to thebattery is high.

When the battery approaches full charge, the battery voltage approachesV_(max), see FIG. 3. The panel 14 then charges the battery with almostthe optimum power, according to FIG. 2. The panel 13 has a diode 23connected in series with the battery. The voltage drop across the diodethen means that the voltage V₃ is higher than V₄, and, according to FIG.2, the current I₃ delivered by the panel is lower than I₄. The currentsfrom the panels 12 and 11 are further reduced in a corresponding manner.

This means that the panels deliver a low maintenance current whichmaintains the charge of the battery 16 and compensates for theself-discharge which normally occurs in the battery. Thus an automaticcurrent limitation is obtained with a high charging voltage at thebattery.

FIG. 5 shows an alternative embodiment of the circuit with seriesconnection of the solar-cell panel 11 and the diodes 21. An arbitrarynumber of diodes 21a, 21b and 21c can be connected in series orconnected completely in shunt by means of a change-over switch with onearm 31 and four contacts 32, 33, 34 and 35. In the Figure, the arm 31 isconnected to the contact 33 which means that the diodes 21a and 21b areby-passed. Current from the panel 11 only flows through the diode 21c.When the arm 31 is connected to the contact 32, all the diodes 21a-21care out of operation and when it is connected to the contacts 34 and 35,the diodes 21b and 21c or 21a, 21b and 21c respectively are connected.

In FIG. 5, a control device 39 is indicated in broken lines and controlsthe change-over switch 30 depending on the charging voltage of thebattery.

The expert can find other methods and devices within the scope of thefollowing claims.

I claim:
 1. A device for charging an electrical accumulator battery bymeans of solar panels containing solar-cells characterised by aplurality of charging branches connected in parallel, each comprising atleast one solar-cell panel, which charging branches contain solar-cellpanels of substantially the same power, and in which branches at leastone diode is respectively connected in series between at least some ofthe solar panels and the battery to constitute the sole switching meansfor disconnecting the solar panels in the respective branchesautomatically in response to variations in battery voltage, the chargingbranches being adapted to deliver charging current at different voltageswith at least one voltage from one of the panels by means of the diodesbeing higher than the others.
 2. A device as claimed in claim 1,characterised in that the charging branch which is adapted by the diodesto deliver the highest voltage, is adapted to deliver a voltage which issomewhat higher than the voltage of the fully charged accumulatorbattery.
 3. A device as claimed in either claim 1 or 2, characterised inthat all the charging branches are adapted by the diodes to deliversuccessively higher voltages.